Electric precision injection unit

ABSTRACT

An injection molding device with an injection molding cylinder in which is arranged a coaxial worn that is rotatable and axially displaceable by drive devices. A spindle nut rotatable by a first drive device is mounted in a housing and cooperates with a screw sleeve which is axially displaceable during rotation of the spindle nut but is fixed against rotation during axial displacement. Further, a shaft is mounted in the interior of the screw sleeve and is connected at one end to the worm and has at the other end an axial coupling, one of whose coupling parts communicates with a second drive device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to a precision injection unit for a machinefor producing molded articles with a drive device for the rotation ofthe worm and a drive device for the axial movement of the worm.

2. Description of the Related Art

DE 42 06 966 discloses an injection unit with only one drive motor forthe axial and radial movement of the worm. The rotating movement of theworm is carried out by the motor via a belt drive, one belt drive beingconnected with the worm shaft by a splined shaft section in a positiveengagement, but so as to be axially displaceable.

The rear part of the worm shaft is constructed as a ball spindle andengages with the corresponding spindle nut. The ball spindle nut issupported in the frame so as to be rotatable and can be secured by aclaw coupling relative to the frame. A spring element constantly pressesthe ball spindle with the spindle nut axially against the frame.

During the plastifying process for plastic, the coupling is not engaged.During the injection process, the coupling is activated, so that theworm is compelled to move axially when the motor rotates. Only a veryslight small axial lift or stroke is achieved in this construction.

Further, it must be viewed as a disadvantage that the worm rotation andthe axial position can not be influenced independent from one another.

EP 0 427 866, to which U.S. Pat. No. 5,129,808 corresponds, describes aninjection unit with a metering motor and an injection motor which isconstructed as a dual-platen injection unit. The injection unit has afixed platen in which two ball spindles arc secured axially and amovable platen with two linear guides which are connected with oneanother via the ball spindles and associated ball spindle nuts. Themetering motor is fastened to the movable platen and drives the worm (inrotation) via a belt drive.

The injection motor is fastened to the fixed platen and drives the twoball spindles via a belt drive for the axial injection movement of themovable platen.

The described injection unit has a complicated mechanical constructionand, accordingly, a great many movable elements which results inincreased maintenance and wear. Since the injection motor must move verymassive parts of the injection unit, the mass moment of inertia is alsohigh and accordingly limits effectiveness and efficiency.

SUMMARY OF THE INVENTION

Proceeding from the problems and disadvantages described above, theobject of the invention is to provide an above-average, economicallyoperating and reliable precision injection unit using features which areknown in part.

An extremely compact construction of the drive block of the injectionunit which is limited to a minimum of parts is achieved by integratingtwo drive devices for axial movement and worm rotation, wherein it ispossible to achieve a temporary flow of force. High efficiency and highavailability are achieved in this way.

A drive shaft with a cylindrical connection for the worm coupling ismounted so as to be freely rotatable at the other end along with asplined shaft section in a screw sleeve with two axial load-bearingcapability rolling bearings. The splined shaft section of the shaftengages with a complementary axial splined shaft coupling which isconnected, via a gear unit, to the servo motor for the rotating movementof the worm.

The screw sleeve, preferably a planetary roll spindle, engages with acomplementary spindle nut. The spindle nut is freely rotatable in thehousing of the drive block of the injection unit with two axialload-bearing rolling bearings. A servo motor drives the spindle nut bymeans of a belt drive and, depending on the rotating direction, thescrew sleeve and, therefore, also the plastifying worm moves axially inone or the other direction, since the screw sleeve operates as a meansfor preventing rotation which is guided in a housing groove andtherefore prevents the screw sleeve from participating in rotation.

The axial movement (injection/metering or influencing of the pressureprofile of the melt) and the rotation of the worm (plastifying) can becarried out completely independent from one another.

The shortest possible flow of force with the smallest mass moment ofinertia combined with high efficiency and low maintenance is achieved bymeans of this construction.

Only the selected construction length of the screw sleeve, and thelength of the splined shaft profile coupling which is adapted to it,limits (a drive block) the possible injection stroke of the worm.

An embodiment example of the invention is shown in the drawings anddescribed in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified view of the drive block of the injection unit;

FIG. 2 shows a practical construction;

FIG. 3 shows a section along line A—A.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The shaft 1 has a cylindrical part 16 for the connection of the wormcoupling, not shown, at one end and a splined shaft profile at the otherend. The shaft 1 is supported in the screw sleeve 8 by the shaftbearings 2 and can rotate independent from it. The radial forces and thehigh axial forces occurring particularly during injection are conductedinto the screw sleeve 8 by the shaft bearings 2.

The axial splined shaft coupling 3 engages with the splined shaftprofile of the shaft 1. The axial splined shaft coupling 3 is fixedlyconnected with the drive shaft of the gear unit 4 which is screwed tothe housing 11. The torque of the electric motor 7 which is alsofastened to the housing 11 Is transmitted to the gear unit 4 via thebelt drives 5 and 6.

The spindle nut 9 engages with the screw sleeve 8 and is supported inthe housing 11 by the spindle nut bearing 10 so as to be rotatable. Thehigh axial forces occurring during the injection are conducted into thehousing 11 via the spindle nut bearing 10.

The spindle nut 9 has a flange to which the belt drives 13 and 14 areflanged and the torque can accordingly be transmitted from the electricgear unit motor 15 fastened to the housing 11 and from the belt drive.

The housing 11 has a block 12 which is received in an axially extendinggroove in the screw sleeve 8 and prevents the screw sleeve 8 fromrotating along during rotation of the spindle nut 9 and shaft 1.

When the spindle nut 9 is set in rotation, the screw sleeve 8 mustcompulsorily move with the shaft 1 in axial direction.

A pressure sensor is provided at a bearing loaded by axial force fordirect instantaneous measurement of axial force.

The arrangement and construction of the drive devices and belt drivesare shown particularly in the practical construction shown in FIGS. 2and 3.

I claim:
 1. An injection molding apparatus comprising a housing, a screwsleeve which is axially displaceable with respect to said housing, meansfor preventing rotation of said screw sleeve with respect to saidhousing, a shaft which is mounted concentrically in said screw sleeveand is axially fixed but rotatable with respect to said screw sleeve,said shaft having one end for carrying a worm in a molding cylinder andan axially opposed end, a spindle nut which cooperates with said screwsleeve to axially displace said screw sleeve with respect to saidhousing when said spindle nut is rotated, a first drive device forrotating said spindle nut, thereby axially displacing said shaft, anaxial coupling on said opposed end which is rotatably fixed but axiallydisplaceable with respect to said opposed end, and a second drive devicefor rotating said axial coupling, thereby rotating said shaft.
 2. Aninjection molding apparatus as in claim 1 wherein each of said first andsecond drive devices comprises an electric motor.
 3. An injectionmolding apparatus as in claim 2 wherein each said electric motor is aservo motor.
 4. An injection molding apparatus as in claim 1 whereinsaid opposed end of said shaft is splined.
 5. An injection moldingapparatus as in claim 1 wherein said screw sleeve is constructed as aball roll spindle.